A critical review of risk matrices used in water safety planning: improving risk matrix construction.

Risk matrices are used in water safety planning to prioritize improvements to drinking water systems. While water safety plans (WSPs) are promoted globally, no study has evaluated whether risk matrices are comprehensively constructed to accurately assess risk. We used risk matrix criteria adapted from previous risk matrix research to evaluate risk matrices found in twelve templates across global jurisdictions. WSP templates were found using the WSPortal website and definitions of likelihood and impact were extracted from each template to assist in the evaluation of WSP risk matrices. Application of the criteria developed from a detailed mathematical analysis by revealed that 11 of 12 risk matrices evaluated contravene at least one of the risk matrix criteria. Furthermore, definitions of likelihood and impact varied widely across different jurisdictions, due in part to the system specific nature of the WSP methodology. To improve risk matrix construction, we recommend: setting clearer risk level boundary criteria, aligning specific impact category definitions with water system objectives, and selecting specific impact categories as opposed to defining impact in several ways. Finally, we recommend risk matrix construction be reviewed as part of the WSP process to ensure accurate identification of key risks in a water system.